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Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Qualified Plan
Share-based compensation arrangement by share-based payment award
Eligible contribution percentage	4.00%	3.00%	2.00%
Percentage of common shares outstanding	3.00%	3.00%
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Defined Contribution Plan, Cost Recognized	$ 6.3	$ 4.8	$ 6.3

X

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary.

+ References

No definition available.

+ Details

Name:	wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the percentage of total common shares outstanding owned by the plans.

+ References

No definition available.

+ Details

Name:	wtm_PercentageOfCommonSharesOutstandingOwnedByPlans
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration